The Sun's atmosphere is filled with ultrasound-like waves that may help solve decades-old mysteries about the Sun and space weather, according to scientists at Southwest Research Institute's Boulder office, who found the waves in data from NASA's TRACE spacecraft. The result has been published in the December 10th, 2004 issue of Astrophysical Journal Letters.
Dubbed ``solar ultrasound'', the waves are actually about 300 times deeper than the deepest pitch audible to the human ear, at a frequency of 100 milliHertz (10 second period).
``At 10 second period, these waves qualify as `ultrasound' because individual atoms on the Sun only experience a few collisions during the brief passage of each wave, just as with ultrasound here on Earth'', said Dr. Craig DeForest, who found the signature in TRACE data collected in January 2003.
The waves are most likely created by the sudden collapse of magnetically induced electric currents (``magnetic reconnection''), or by lower frequency sound waves that crash like ocean waves as they make their way up from the surface of the Sun. Both of those sources are likely candidates for the source of the solar atmosphere's mysterious extra heat, making the new waves a valuable new tool for exploring a decades-old mystery.
At up to 100,000 degrees C (180,000 degrees F), ``chromosphere'', or middle solar atmosphere, is nearly twenty times times hotter than the 6,000 C (11,000 F) surface of the Sun. The solar corona, at 1,000,000 C (1,800,000 F) is about 10 times hotter still, or 200 times hotter than the surface of the Sun. Although scientists have been studying the process for over 50 years, the exact reason is still elusive.
``By examining these waves more closely, we should be able to discern the source of energy release in the solar atmosphere", says DeForest, ``just like you can tell by listening whether the car is running in a dark garage. In both cases, something is releasing energy into the environment, and that release has a recognizable sonic signature."
The Sun is filled with lower-pitched waves, at about 3 milliHz (5 minute period) that are used to probe the solar interior and even to make images of the far side of the Sun. The solar ultrasound is too high pitched to be directly related to these more well known "photospheric oscillations".
Sound waves cannot travel through interplanetary space, so they are detected remotely, as small fluctuations in the brightness of solar ultraviolet emission. The TRACE spacecraft, built by Lockheed-Martin for NASA's Explorer program, is an ultraviolet telescope in orbit around Earth. The solar ultrasound is right at the limit of detectability by TRACE, so faint that individual waves cannot be resolved. Instead, DeForest sleuthed for patterns in the background noise of the telescope.
``Each individual wave train has an amplitude about 1/10 of the smallest brightness value that TRACE can see'', said DeForest. ``But when we average many images together in the right way, a pattern emerges that we can recognize as the signature of trapped waves''. The pattern emerges through three-dimensional Fourier analysis, a technique that isolates individual types of motion from the morass of activity that characterizes solar movies.
The pattern, barely detectable with TRACE, exists because the waves appear to be trapped in a thin layer of the solar atmosphere, which imprints a particular signature pattern onto the sound -- just as the acoustics of a concert hall affect the sound of a symphony. TRACE produces a digital movie of the surface of the Sun. Scientists then process the movie through a mathematical process called "Fourier analysis", to produce a wave diagram like the one shown above, right. The waves appear as sloped ridges in the wave diagram, showing the presence of soundlike waves in the octave between 50 milliHz and 100 milliHz.
Although the waves appear faint, they are quite energetic. ``These ripples seem to be carrying about 1 kilowatt of power per square meter on the surface of the Sun'', says DeForest. ``That is similar to the sonic energy you might find coming out of the speakers at a rock concert. Very loud.''
``The discovery of coherent waves at such high frequencies in the upper solar atmosphere challenges our understanding of the magnetic structures in the quiet Sun'', said Dr. Joseph Gurman, TRACE mission scientist at NASA's Goddard Space Flight Center. ``This work proves that we need new tools to understand the propagation of energy into and out of this part of the solar atmosphere -- where much of the activity that can affect life here on Earth originates.''
Future instruments will be able to detect the waves better: while TRACE is a simple telescope that can only detect changes in brightness, the waves are likely to have a much stronger Doppler signature. One problem: Earth's ozone layer is opaque to the ultraviolet light used to see and ``hear'' the solar ultrasound, making it difficult to detect from the ground. SwRI scientists are designing rocket- and balloon-borne instruments to observe from above the bulk of Earth's atmosphere, to catch a better glimpse of the wave spectrum and exploit it to probe the solar atmosphere.